"We have a significant line of inquiry which is worth pursuing and because it's worth pursuing it could provide an answer - but until we've gone through it I won't know whether we are going to get there or not."

Police will give no details of the lead - nor why it is proving so difficult to resolve. But they want more time to pursue it.

There has been speculation that it involves a hunt for a suspect or a key witness, but none of that has been confirmed.

Image:The McCanns were staying on the ground floor of this apartment block

Retired Metropolitan Police detective Peter Kirkham said: "Media interest and the political drive for it to be done in the first place will add to a reluctance to stop the investigation until they are absolutely sure they have done everything they can.

"If a line of inquiry has any signs of life in it whatsoever, they will be saying 'this is here and we can do it but we'll need this amount to do it and if you don't wish to pay for it that is your decision'."

A spokesman for Madeleine's family told Sky News: "The McCanns will be pleased. The fact that the police feel there is still work to be done which needs funding is very encouraging.

"They remain grateful to all the officers involved in the investigation."

Image:Kate and Gerry with a photo of how Madeleine might have looked aged nine

Scotland Yard said: "Funding is in place until the end of September. Any details about future funding will be released when appropriate."

The Home Office confirmed that funding for Operation Grange had been provided until the end of next month.